(1) Recombinant plasmids for the expression of polyoma virus middle tumor antigen (MT) were constructed in order to obtain a large quantity of MT and to study its properties. To achieve a high level of expression of the MT gene in E. coli, a strong promoter, PL, of lambda phage, whose activity is controlled by the temperature sensitive cI gene product was used. Immunoprecipitable proteins of 50K, 53K, and 55K molecular weight were produced and the 53K and 55K proteins were identified as MT by MT specific monoclonal antibodies. They did not react with large T-specific monoclonal antibodies. [35S]methionine-labeled 55K protein is indistinguishable in peptide mapping from that of authentic middle T antigen. (2) Based on a hypothesis that metallothionein (MET) or metal-free thionein (apothionein) plays a role in the regulation of the expression of the MET gene, studies are progressing to observe the interaction of apothionein with the control region of the human MET gene. Plasmid DNA containing the human MET gene and apothionein from HeLa cells was isolated. Interactions between apothionein and DNA are studied by DNA affinity column chromatography, DNase protection studies and in vitro transcription experiments. (3) Plasmid vectors containing bovine papilloma virus (BPV), MET promoter-operator regions and mouse Beta-interferon gene coding regions are also constructed in such a way that metal-inducible interferon may interact with BPV transforming activity in the cells transfected with the cloned plasmid. These studies will give insight into the molecular and genetic mechanisms of gene expression associated with the oncogenic processes.